Effects of nitrate and ammonium supply ratios on growth, physiology and yield of oilseed rape (Brassica napus L.)

Objectives A reasonable application of nitrogen fertilizer plays an important role in improving oilseed rape cultivation and promoting oilseed yield. Therefore, the application effects with different ratios of nitrate and ammonium on the growth, physiological response and oilseed yield of oilseed rape throughout the growth process including seedling stage, flowering period, and harvesting period.

Methods Two cultivars of oilseed rape (Brassica napus L.), Xiangyou15 with high N efficiency and 814 with low N efficiency, were cultured in sands under supply with the same N rate. Five treatments, which represent different mole ratios of NO₃^– versus NH₄⁺ 100/0(N1), 75/25(N2), 50/50(N3), 25/75(N4), and 0/100(N5), were established. The dry weight, root configuration, photosynthesis, N concentration, oilseed yield, and oil contents in oilseed rape were analyzed at different growth stages.

Results Both the N1 and the N2 treatments demonstrated more increases in dry weight, root length, root surface, root volume, grain yield, oil content, and oil yield of oilseed rapes than the other treatments with nitrogen fertilizer during the whole growth period, while the treatment with N5 inhibited the growth of oilseed rapes. At the seedling stage, the SPAD values in leaf in the treatments with N2, N3 and N4 were significantly higher than those in the treatments with N1 or N5. Additionally, in the case of supply with N1, N2, N3 and N4, the root length, root surface, root volume, grain yield, oil content, and oil yield in Xiangyou15 variety with high N efficiency were significant higher than those in cultivar 814 with low N efficiency during the whole growth period. Xiangyou15 cultivar accumulated more dry weights than 814 variety at the harvesting period when the treatments were supplied with N1, N2, N3 and N4, but two cultivar varieties had no significant differences in accumulation of N in oilseed rapes.

Conclusions A suitable ratio of nitrate (75% NO₃^–) and ammonium (25% NH₄⁺) could promote photosynthesis and improve the growth of oilseed rape and oilseed yield. Compared with 814 variety, the xiangyou15 variety with higher N efficiency revealed a higher root length, root surface, root volume and utilization of nitrate.